going by category will be problematic.
I would add another column to the products table and the cart object for Discountable.
for items that belong to the categories that the discount should apply to set the discountable column in the products table to 1.
in your add to cart button behaviors, bind the Discountable cart column to the Discountable column of the products table>
then in your discount, you can add a trigger for:
Subtotal for any column:
if the subtotal for column "Discountable" is ">" a value of "0"
and if the calculation is a percentage, use:
Items with a specific value:
total of "Total Price" where "Discountable" has a value of "1" "times" "0.1"
NOTE: when adding a column to the cart object, you will need to edit each add to cart behavior to have the code regenerated.
